CT scanners are used for medical imaging as well as for industrial inspection applications. CT imaging is based on an x-ray source mounted on a rotatable gantry opposite an x-ray detector array. The x-ray source directs an x-ray beam in a direction perpendicular to a rotational axis of the gantry through an object and toward the x-ray detector array as the gantry is rotated. The object is typically centered about the rotational axis of the gantry. The directed x-ray beam partially attenuated by the object impinges on the x-ray detector array to form an x-ray image of the object. Rotating the gantry provides for acquiring a plurality of angular views of the object. Typically an angular span of about 180 degree or more is used for imaging. In helical scanning, the rotating gantry rotates as the object is relatively translated along the rotational axis of the gantry. An image reconstruction processor typically uses filtered back projection or other reconstruction techniques to produce a reconstructed volume image based on the acquired x-ray images.
Fan beam CT scanners employ an x-ray fan beam and a linear detector array. In fan beam CT scanning, a single two-dimensional (2D) image data set, e.g. slice of an object is captured for each rotation of the gantry. Acquiring a 3D data set, one 2D slice at a time is inherently slow. Moreover, in medical applications, motion artifacts occur because adjacent slices are not imaged simultaneously. Also, dose utilization is less than optimal, because the distance between slices is typically less than an x-ray collimator aperture used to project the x-ray beam, resulting in double exposure to many parts of the inspected object. Also known are multi-slice CT scanners employing detectors with multiple rows of detector elements that scan multiple slices at a time. Multi-line CT scanners improve some of these shortcomings to a limited degree.
Cone beam CT scanners include an x-ray source that projects a pyramid shaped x-ray beam opposite a 2D detector array for detecting an area image of the object. With cone beam CT scanners a larger volume of the inspected object is covered over one rotation of the gantry as compared to a fan beam system. With the larger volume coverage per rotation, motion artifacts can be reduced as well as the scanning period required to cover the object. One of the known drawbacks of cone beam CT scanning, as will be described in further detail below, is that the data set collected over a single scan is incomplete. Missing data introduces artifacts during image reconstruction, resulting in images which may be inadequate for, for example, medical diagnosis or part quality determination purposes.
Tuy H., “An inversion formula for cone-beam reconstruction”, SIAM Journal of Applied Mathematics 43, 546-552” (1983), the contents of which is incorporated herein by reference, describes a condition for achieving data completeness in cone beam CT scanning that has been widely accepted. In what is known as the Tuy-Smith condition, data completeness is achieved when the source trajectory intersects every plane passing through the reconstructed volume in the scanned object.
A number of source trajectories have been attempted to increase the field of view of the CT scanner and to reduce cone-beam artifacts.
U.S. Pat. No. 5,784,481, entitled “CT cone beam image reconstruction with circle and line scan path” the contents of which is incorporated herein by reference, describes a CT cone beam imaging system that provides relative movement between the cone beam source and the object along a scan path resulting in a helical scan. The relative movement can be provided by linear movement of a patient support member or alternatively by translation of the gantry.
U.S. Pat. No. 7,305,063, entitled “Cylindrical x-ray tube for computed tomography imaging”, the contents of which is incorporated herein by reference, describes an x-ray tube with a cylindrical anode that rotates about a longitudinally aligned cylinder axis. Electrons are accelerated toward a selected spot on a target outer surface region of the cylindrical anode. Electrostatic or electromagnetic deflectors sweep the selected spot back and forth across the target outer surface region of the cylindrical anode providing an axial beam sweeping. This provides for distributing heating across a cylindrical anode to improve thermal characteristics of the x-ray tube. One embodiment describes using the beam sweep to provide longitudinal scanning while the subject support remains stationary in cardiac imaging. It is stated that the beam sweep should be coordinated with the angular rotation of the gantry to ensure sufficient angular coverage for each voxel in the image volume. The trajectory described is a helical trajectory along a 12 cm length.
U.S. Pat. No. 6,504,892, entitled “System and method for cone beam volume computed tomography using circle-plus-multiple-arc orbit” the contents of which is incorporated herein by reference, describes a system and method for cone beam volume computed tomography by taking signals along an orbit having a circle plus two more off-axis arcs. The arc data is used to reconstruct data that cannot be recovered from the circle scan.
U.S. Pat. No. 5,712,889 entitled “Scanned volume scanner” the contents of which is incorporated herein by reference, describes a volume CT scanner with a means for generating an electron beam and linearly scanning it along a target to form a succession of cone-shaped x-ray beams. A vane collimator receives the successive cone-shaped x-ray beams and forms a succession of adjacent parallel fan-shaped x-ray beams which successively irradiate adjacent planar slices along the length of a volume of the object to be scanned. A detector array includes a plurality of adjacent longitudinally elongated detector elements extending across each of said beams for receiving the succession of transmitted parallel fan-shaped x-ray beams.
U.S. Pat. No. 5,068,882, entitled “Dual parallel cone beam circular scanning trajectories for reduced data incompleteness in three-dimensional computerized tomography”, contents of which is incorporated herein by reference, describes a CT cone beam imaging system for minimizing the incompleteness of the data set acquired. Among other embodiments there is described a system that reduces data incompleteness by using two cone beam x-ray sources offset from each other by 90 degrees with corresponding area detectors. The two sources form two parallel circular trajectories during gantry rotation.
US Patent Application Publication No. 20060285633, entitled “Multiple source CT scanner”, the contents of which is incorporated herein by reference, describes a CT scanner including a plurality of cone-beam x-ray sources offset along a CT axis, the axis over which the x-ray source and detector are rotated. A detector is positioned opposite the x-ray sources. The x-ray sources and detector are rotatable about the CT axis. The x-ray sources direct x-rays through the patient that are received by the detector at a plurality of rotational positions, thereby generating projections from the plurality of x-ray sources that are used to construct the three-dimensional CT image of the patient.
International Patent Application Publication No. WO 2006/038145, entitled “Computed Tomography Method”, the contents of which is incorporated herein by reference, describes a computed tomography method and apparatus where a focal spot of a cone beam is switched between at least two positions spaced apart from each other and arranged on a line parallel to the axis of rotation to enlarge the reconstructable examination zone parallel to the axis of rotation.